The present invention relates to a continuous carburizing furnace which performs a plurality of processes, including a carburizing process, successively upon a workpiece, the subject for processing, which is being conveyed in an ambient atmosphere which includes a carburizing gas.
With a continuous carburizing furnace, a heating zone, a carburizing zone, a diffusion zone, a cooling zone, and so on are provided within the furnace. A workpiece which has been loaded upon a tray is subjected to processing in each of these zones, while the tray is conveyed from a transport entrance of the furnace towards a removal aperture thereof.
As methods for conveying the workpiece within the furnace, both the tray pusher method and the roller hearth method are available. With a continuous carburizing furnace which utilizes the tray pusher method, as for example disclosed in Japanese Laid-Open Patent Publication 2004-10945, a tray most to the upstream side is pushed by a pusher from the transport entrance towards the removal aperture, and thereby a plurality of trays are conveyed while being kept in mutual contact. On the other hand, with a continuous carburizing furnace which utilizes the roller hearth method, a large number of hearth rollers which are arranged across the floor of the furnace are rotationally driven, so that the trays are shifted over these hearth rollers.
It is necessary to apply mutually different levels of heating energy to the heating zone and to the carburizing zone within the furnace. Furthermore, the carburizing zone receives an input of a carburizing gas. In order to enhance the product quality of the workpiece after carburizing processing, it is necessary to keep the temperature and the ambient atmosphere in each zone constant; and, to this end, it has been contemplated to selectively isolate the heating zone, in which the temperature differences with the previous and successive zones are most conspicuous, with intermediate doors which are opened and closed as required.
With the roller hearth method, it is possible to adjust the gaps between the various trays in a simple and easy manner by controlling the rotation of the hearth rollers. Due to this, continuous carburizing furnaces which utilize the roller hearth method, and in which intermediate doors are installed between the heating zone and the carburizing zone, are nowadays widespread.
However, with a continuous carburizing furnace which utilizes the roller hearth method, it is necessary to drive the large number of hearth rollers from the exterior, and a considerable amount of thermal energy is wasted by thermal diffusion from the side walls of the furnace in which the shafts of the hearth rollers are supported. Furthermore, it becomes necessary to oscillate the hearth rollers by rotating them forwards and backwards periodically in order to prevent deflection of the hearth rollers due to the loadings imposed upon them from the trays, so that the drive control of the rollers becomes troublesome. Moreover, the maintenance of this large number of hearth rollers also becomes complicated and troublesome. Yet further, the size of the furnace is increased due to the provision of the gaps between the plurality of trays.
On the other hand, with a continuous carburizing furnace which utilizes the tray pusher method, it is possible to eliminate the above described shortcomings of the roller hearth method; and, by changing the stroke of the pusher, it is possible to provide a gap between the tray which is most towards the upstream side and the tray in front of it. However, a purge chamber which is provided with an intermediate door between itself and the heating zone is present at the transport entrance side of the furnace, and it is not possible to bring in the next tray to this purge chamber until the previous tray has been conveyed from the heating zone to the carburizing zone, so that the time period between bringing in trays becomes long.
Moreover, by providing a plurality of pushers whose pushing angles in plan view are mutually orthogonal, and by changing the direction of conveyance of the trays within the furnace in a zigzag manner, it is possible to create a gap between a pair of trays, during their passage through the furnace. However, in this case, the shape of the furnace in plan view cannot be made to be linear, so that the area which the device occupies is increased in size.
The objective of the present invention is to supply a continuous carburizing furnace which operates according to the tray pusher method, with which, while maintaining the shape in plan view of the conveyance path as being a straight line, the conveyance path for trays with workpieces loaded upon them is made to include a plurality of stages at the upstream side of the carburizing zone, and with which, by providing a plurality of pushers which push the trays at each stage, it is made possible to establish gaps between each of the trays being successively conveyed and the next one, in order to allow the operation of intermediate doors which are installed.